CD4 and infection: Because HSPCs generally have limited surface expression of CD4, their abnormalities in HIV infection could be largely explained as an indirect effect of HIV infection, rather than the results of direct infection of HSPCs (Louache et al., 1992; De Luca et al., 1993; Maciejewski et al., 1994; Marandin et al., 1996; Koka et al., 1999).